Wire bonding method

ABSTRACT

A wire bonding method includes the following steps. First, a substrate including at least one metal finger is provided. Next, a first chip including at least one first boding pad is disposed on the substrate. Next, a metal ball bump is formed on the corresponding metal finger. Next, a first wire is formed from the metal ball bump toward the corresponding first boding pad. Next, a first free air ball is formed on the first wire by electronic flame-off process. Then, the first free air ball connected to the first wire is pressed on the corresponding first boding pad, such that the first wire is located between the first free air ball and the corresponding first boding pad.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of and claims the priority benefit ofpatent application Ser. No. 14/620,947, filed on Feb. 12, 2015, now U.S.Pat. No. 9,362,254. The entirety of the above-mentioned patentapplication is hereby incorporated by reference herein and made a partof this specification.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to a wire bonding method and achip structure. More particularly, the present invention relates to awire bonding method and a chip structure using the same.

Description of Related Art

Modern electronic equipment relies heavily on printed circuit boards onwhich semiconductor chips, or integrated circuits (ICs), are mounted.The mechanical and electrical connections between the chip and thesubstrate have posed challenges for chip designers. Wire bonding is oneof the well known techniques for interconnecting the IC to thesubstrate.

In general, wire bonding technique includes “forward bonding” and“reversed bonding”. Forward bonding refers to bonding from an integratedcircuit/component down to a substrate. A forward bonding process placesa ball bond on the die first, a capillary then forms a stitch bond fromthe ball bond to a pad of the substrate. Most wire bonding applicationsuse the typical forward bonding process, because it is faster and morecapable of finer pitch than reverse bonding. However, forward bondinghas a loop height constraint due to the neck area above the ball.Excessive bending above the ball can cause neck cracks, which results inreliability problems. Therefore, the chip structure using forwardbonding process is hard to meet the requirements of low-profile looping.

A reverse bonding process or so called Stitch-bond Stands on Bump (SSB),on the contrary, places a bump on the die pad first. After the bump isformed, a ball bond is placed on the substrate, followed by a stitchbond on the bump. With this metal ball bump on the pad of chip, it cancreate buffer to keep the high down force of stitch bond from damagingthe underlayer structures beneath the pad. Low-profile loopingrequirements have propelled the growing use of reverse ball bonding.However, reverse bonding is a discontinuous process which is a muchslower process than forward bonding. Therefore, how to meet thelow-profile looping requirements but also keep efficient in manufactureis an important research topic in current industry.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a wire bonding methodwhich has simple process and is more efficient, and the chip structureusing the wire bonding method can meet the low-profile loopingrequirements.

The present invention is further directed to a chip structure whichmeets the low-profile looping requirements and is more efficient inmanufacture.

The present invention provides a wire bonding method including thefollowing steps. First, a substrate including at least one metal fingeris provided. Next, a first chip including at least one first boding padis disposed on the substrate. Next, a metal ball bump is formed on thecorresponding metal finger. Next, a first wire is formed from the metalball bump toward the corresponding first boding pad. Next, a first freeair ball is formed on the first wire by electronic flame-off process.Then, the first free air ball connected to the first wire is pressed onthe corresponding first boding pad, such that the first wire is locatedbetween the first free air ball and the corresponding first boding pad.

The present invention provides a package structure. The packagestructure includes a substrate, a first chip, at least one metal ballbump, at least one first wire and at least one first free air ball afterbonded. The substrate includes a first surface and at least one metalfinger disposed on the first surface. The first chip is disposed on thefirst surface and includes a first active surface and at least one firstboding pad disposed on the first active surface. The metal ball bump isdisposed on the corresponding metal finger respectively. The first wireis connected between the corresponding metal finger and thecorresponding first boding pad respectively. The first free air ball isconnected to the corresponding first wire and disposed on thecorresponding first boding pad respectively, such that the first wire islocated between the corresponding first free air ball and thecorresponding first boding pad.

According to an embodiment of the present invention, the first wire isformed by a capillary of a wire-bonding apparatus moving from the metalfinger toward the corresponding first boding pad.

According to an embodiment of the present invention, the electronicflame-off process further includes the following steps: an electronicflame-off wand is placed with a predetermined distance from the firstwire to create an electrical arc between the electronic flame-off wandelectrode and the first wire.

According to an embodiment of the present invention, the first wire andthe first free air ball are integrally formed.

According to an embodiment of the present invention, the wire bondingmethod further includes the following steps: a second chip is stacked onthe first chip, and the second chip exposes the first boding pad andincludes at least one second boding pad.

According to an embodiment of the present invention, the wire bondingmethod further includes the following steps: after the first free airball is pressed on the corresponding first boding pad, a second wire isformed from the corresponding first boding pad toward the correspondingsecond boding pad. Next, a second free air ball is aimed on the secondwire by electronic flame-off process. Next, the second free air ballconnecting the second wire is pressed onto the corresponding secondboding pad, such that the second wire is located between the second freeair ball and the corresponding second boding pad.

According to an embodiment of the present invention, the second wire isformed by a capillary of a wire-bonding apparatus moving from thecorresponding first bond pad toward the corresponding second boding pad.

According to an embodiment of the present invention, the electronicflame-off process further includes the following steps: an electronicflame-off wand is placed with a predetermined distance from the firstwire to create an electrical arc between the electronic flame-off wandand the second wire.

According to an embodiment of the present invention, the first free airball and the second wire are continuously and integrally formed.

According to an embodiment of the present invention, the second wire andthe second free air ball are integrally formed.

According to an embodiment of the present invention, the distance fromthe first active surface to the highest point of the first wire rangesfrom a diameter of the first wire to 200 μm.

According to an embodiment of the present invention, the distance fromthe second active surface to the highest point of the second wire rangesfrom a diameter of the second wire to 200 μm.

Based on the aforementioned description, the free air ball is formed onthe wire by electronic flame-off process and then pressed onto the bondpad, such that the wire is located between the free air ball and thebond pad. With the wire bonding method described above, the wire bodingof the chip structure can be performed continuously without having toplace a bump on the bond pad first, and then place a metal ball bump onthe metal finger of substrate. Therefore, manufacturing process of thechip structure can be simplified, so as to speed up the wire bondingprocess of the chip structure.

In addition, since the highest point of the wire loop is adjacent to thefirst bond, the wire bonding method of the present invention set thefirst bond to be the lower bonding surface, so as to reduce the overallheight of the wire loop. Moreover, since the wire is located between thefree air ball and the bond pad instead of being disposed on top of thefree air ball, the overall height of the wire loop of the chip structurecan be further reduced. Therefore, the wire bonding method and the chipstructure using the same not only can meet the low-profile requirements,but also can improve the efficiency of the wire bonding process.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1A to FIG. 1H are schematic views illustrating a wire bondingprocess according to an exemplary embodiment.

FIG. 2 to FIG. 3 are schematic views illustrating a process of forming afree air ball on a wire according to an exemplary embodiment.

FIG. 4 is schematic partial-enlarged view of the chip structure in FIG.1F according to an exemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. In this regard, directionalterminology, such as “top”, “bottom”, “front”, “back”, etc., is usedwith reference to the orientation of the Figure(s) being described. Thecomponents of the invention can be positioned in a number of differentorientations. As such, the directional terminology is used for purposesof illustration and is in no way limiting. On the other hand, thedrawings are only schematic and the sizes of components may beexaggerated for clarity. It is to be understood that other embodimentsmay be utilized and structural changes may be made without departingfrom the scope of the invention. Also, it is to be understood that thephraseology and terminology used herein are for the purpose ofdescription and should not be regarded as limiting. The use of“including”, “comprising”, or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless limited otherwise, the terms“connected”, “coupled”, and “mounted” and variations thereof herein areused broadly and encompass direct and indirect connections, couplings,and mountings. Accordingly, the drawings and descriptions will beregarded as illustrative in nature and not as restrictive.

FIG. 1A to FIG. 1H are schematic cross-sectional views illustrating awire bonding process according to an exemplary embodiment. In thepresent embodiment, the wire bonding method includes the followingsteps. Firstly, referring to FIG. 1A, a substrate 110 is provided. Thesubstrate 110 may include a first surface 112 and at least one metalfinger 114. It is noted that a plurality of metal fingers 114 areillustrated in the present embodiment, but the invention is not limitedthereto. The metal fingers 114 are disposed on the first surface 112.Also, a first chip 120 is disposed on the substrate 110. In the presentembodiment, the first chip 120 is attached to the substrate 110 via anadhesive material 125, but, of course, the present invention is notlimited thereto. The first chip 120 may include a first active surface122 and at least one first boding pad 124. It is noted that a pluralityof first boding pads 124 are illustrated in the present embodiment, butthe invention is not limited thereto. The first boding pads 124 aredisposed on the first active surface 122. Of course, the presentembodiment is merely for illustration. In other embodiment, another chipcan be further stacked on the first chip 120. The present invention doesnot limit the number of the chips stacked on the substrate 110.

Referring to FIG. 1B, in the present embodiment, a second chip 160 isstacked on the first chip 120, and a third chip 190 is stacked on thesecond chip 160. The second chip 160 exposes the first boding pads 124and includes a second active surface 162 and at least one second bodingpad 164 disposed on the second active surface 162. The third chip 190exposes the first boding pads 124, second bonding pads 164, and includesat least one third boding pad 194. Then, metal ball bump 130 is bondedon one of the metal fingers 114 of the substrate 110. The metal ballbump 130 can be bonded on the metal fingers 114 by, for example, thermaland ultrasonic energy, etc. The present invention is not limitedthereto.

Referring to FIG. 1C and FIG. 1D, a first wire 140 is formed from themetal ball bump 130 toward one of the first boding pads 124. In detail,the capillary 10 of the wire-bonding apparatus travels from the metalball bump 130 to the first boding pads 124 to form the first wire 140.Then, a first free air ball 150 is formed on the first wire 140 byelectronic flame-off process (EFO). The processes will be described inmore detail below.

FIG. 2 to FIG. 3 are schematic views illustrating a process of forming afree air ball on a wire according to an exemplary embodiment. Referringto FIG. 2 and FIG. 3, the first free air ball 150 formation process isachieved by ionization of an air gap by electronic flame-off (EFO)process. In EFO process, electric heating discharge occurs between twoelectrodes: one electrode may be the first wire 140, typically copper orgold (anode) and the other electrode may be an electrical flame-off(EFO) wand 20 (cathode). To be more specific, the first free air ball150 is formed by placing an electrical flame-off (EFO) wand 20 apredetermined distance from the first wire 140 as shown in FIG. 2, suchthat an electrical arc 15 is formed between the first wire 140 and theEFO wand 20. Accordingly, the heat produced by an EFO wand 20 during thedischarge causes the first wire 140 to melt and surface tension of themolten metal causes the metal to roll up into a ball shape. When asufficient amount of the metal (i.e. first wire 140) has melted, thedischarge is terminated so as to form the first free air ball 150 on thefirst wire 140 as shown in FIG. 3. As such, the first wire 140 and thefirst free air ball 150 are integrally formed. By varying the intensityand the duration of the electrical arc, the size of the first free airball 150 that is formed can be adjusted to specific dimensions.

Referring to FIG. 1E and FIG. 1F, the first free air ball 150 connectedto the first wire 140 is then pressed onto one of the first boding pads124 to form a ball bump on the first boding pads 124, such that thefirst wire 140 is located between the first free air ball 150 and theone of the first boding pads 124. Then, the steps shown in FIG. 1C toFIG. 1F can be repeated to continuously wire bond the second chip 160and the third chip 190. In detail, referring to FIG. 1G, a second wire170 is formed from the abovementioned first boding pad 124 toward one ofthe second boding pads 164. To be more specific, the capillary 10travels from the first boding pad 124 toward the second boding pad 164as shown in FIG. 1G to form the second wire 170. Next, a second free airball 180 is formed on the second wire 170 by the same electronicflame-off process described above. Then, the second free air ball 180connecting the second wire 170 is pressed onto the second boding pad164, such that the second wire 170 is located between the second freeair ball 180 and the second boding pad 164. Referring to FIG. 1H, withsimilar process, a third free air ball 192 connecting the second wire196 is formed on the third boding pad 194, such that the third wire 196is located between the third free air ball 192 and the third boding pad194. As such, the first free air ball 150 and the second wire 170 arecontinuously and integrally formed, and the second wire 170 and thesecond free air ball 180 are integrally formed.

FIG. 4 is schematic partial-enlarged view of the chip structure in FIG.1E according to an exemplary embodiment. Referring to FIG. 1H and FIG.4, with the wire bonding method described above, a package structure 100as shown in FIG. 1H and FIG. 4 can be formed. In the present embodiment,the package structure 100 includes a substrate 110, a first chip 120, atleast one metal ball bump 130, at least one first wire 140 and at leastone first free air ball 150. The substrate 110 includes a first surface112 and at least one metal finger 114 disposed on the first surface 112.The first chip 120 is disposed on the first surface 112 and includes afirst active surface 122 and at least one first boding pad 124 disposedon the first active surface 122. The metal ball bump 130 is disposed onthe corresponding metal finger 114 respectively. The first wire 140 isconnected between the corresponding metal ball bump 130 and thecorresponding first boding pad 124 respectively. The first free air ball150 is connected to the corresponding first wire 140 and disposed on thecorresponding first boding pads 124 respectively, such that the firstwire 140 is located between the corresponding first free air ball 150and the corresponding first boding pad 124. With the disposition, sincethe highest point of the wire loop is adjacent to the metal ball bump130, the overall height of the first wire 140 can thus be reduced. Inthe present embodiment, the distance D1 from the first active surface122 to the highest point of each first wire 140 may range from, forexample, a diameter of the first wire 140 to 200 μm.

In addition, the package structure 100 may further includes a secondchip 160, at least one second wire 170 and at least one second free airball 180. The second chip 160 as shown in FIG. 1H may be stacked on thefirst active surface 122 of the first chip 120 and exposing the firstboding pad 124. The second chip 160 includes at least one second bodingpad 164 disposed on the second active surface 162 as shown in FIG. 1B.The second wire 170 is connected between the corresponding first bodingpad 124 and the corresponding second boding pad 164. The second free airball 180 connected to the corresponding second wire 170 is disposed onthe corresponding second boding pad 164 respectively, such that thesecond wire 170 is located between the corresponding second free airball 180 and the corresponding second boding pad 164. The overall heightof the second wire 170 can also be reduced. In the present embodiment,the distance from the second active surface 162 to the highest point ofthe second wire 170 may range from, for example, a diameter of thesecond wire to 200 μm.

In sum, the free air ball is formed on the wire by electronic flame-off(EFO) process and then pressed onto the bond pad, such that the wire islocated between the free air ball and the bond pad. With the wirebonding method described above, the wire boding of the chip structurecan be performed continuously without having to place a bump on the bondpad first, and then place a metal ball bump on the substrate. Therefore,manufacturing process of the chip structure can be simplified, so as tospeed up the wire bonding process of the chip structure.

In addition, since the highest point of the wire loop is adjacent to thefirst bond, the wire bonding method of the present invention set thefirst bond to be the lower bonding surface, so as to reduce the overallheight of the wire loop. Moreover, since the wire is located between thefree air ball and the bond pad instead of being disposed on top of thefree air ball, the overall height of the wire loop of the chip structurecan be further reduced. Therefore, the wire bonding method and the chipstructure using the same not only can meet the low-profile requirements,but also can improve the efficiency of the wire bonding process.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A wire bonding method, comprising: providing asubstrate comprising at least one metal finger; disposing a first chipon the substrate and the first chip comprising at least one first bodingpad; forming a metal ball bump on the corresponding metal finger;forming a first wire from the metal ball bump toward the correspondingfirst boding pad; forming a first free air ball on the first wire byelectronic flame-off process; and pressing the first free air ballconnected to the first wire on the corresponding first boding pad, suchthat the first wire is located between the first free air ball and thecorresponding first boding pad.
 2. The wire bonding method as claimed inclaim 1, wherein the first wire is formed by a capillary of awire-bonding apparatus moving from the metal ball bump toward thecorresponding first boding pad.
 3. The wire bonding method as claimed inclaim 1, wherein the electronic flame-off process further comprises:placing an electrical flame-off wand a predetermined distance from thefirst wire to create an electrical arc between the electronic flame-offwand electrode and the first wire.
 4. The wire bonding method as claimedin claim 1, wherein the first wire and the first free air ball areintegrally formed.
 5. The wire bonding method as claimed in claim 1,further comprising: stacking a second chip on the first chip, and thesecond chip exposing the first boding pads and comprising at least onesecond boding pads.
 6. The wire bonding method as claimed in claim 5,further comprising: after pressing the first free air ball on thecorresponding first boding pad, forming a second wire from thecorresponding first boding pad toward one of the corresponding secondboding pad; forming a second free air ball on the second wire byelectronic flame-off process; and pressing the second free air ballconnecting the second wire onto the corresponding second boding pad,such that the second wire is located between the second free air balland the corresponding second boding pad.
 7. The wire bonding method asclaimed in claim 6, wherein the second wire is formed by a capillary ofa wire-bonding apparatus moving from the corresponding first bond padtoward the corresponding second boding pad.
 8. The wire bonding methodas claimed in claim 6, wherein the electronic flame-off process furthercomprises: placing an electrical flame-off wand a predetermined distancefrom the second wire to create an electrical arc between the electronicflame-off wand and the second wire.
 9. The wire bonding method asclaimed in claim 6, wherein the first free air ball and the second wireare continuously and integrally formed.
 10. The wire bonding method asclaimed in claim 6, wherein the second wire and the second free air ballare integrally formed.